The present invention relates to the communication of digital data using variable length codewords, and more particularly to a method and apparatus for facilitating the recovery from transmission errors affecting the boundaries for data blocks containing variable length codewords.
Television signals are conventionally transmitted in analog form according to various standards adopted by particular countries. For example, the United States has adopted the standards of the National Television System Committee (NTSC). Most European countries have adopted either PAL (Phase Alternating Line) or SECAM (Sequential Color and Memory) standards.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
A substantial amount of digital data must be transmitted in any digital television system. In a digital television system, a subscriber receives the digital data stream via a receiver/descrambler that provides video, audio and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must be transmitted.
The video portion of a television signal comprises a sequence of video "frames" that together provide a moving picture. In an interlaced transmission scheme, each frame is transmitted as two separate "fields," an even field and an odd field, which are interlaced to provide a full video frame. Such interlacing avoids the perception of flicker in the received video image. In the NTSC system, each displayed frame consists of 525 horizontally swept lines. Roughly thirty frames, corresponding to sixty fields, are displayed each second.
In digital television, each line of a video frame is defined by a sequence of digital data referred to as "pixels." A large amount of data is required to define each video frame of a television signal. For example, 5.9 megabits of data is required to provide one video frame at NTSC resolution. This assumes a 512 pixel by 480 line display that is used with eight bits of intensity value for each of the primary colors red, green and blue. High definition television (HDTV) requires even more data to provide each video frame. In order to manage this amount of data, particularly for HDTV applications, the data must be compressed.
Video compression techniques enable the efficient transmission of digital video signals over conventional communication channels. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the important information in a video signal. The most powerful compression systems not only take advantage of spatial correlation, but can also utilize similarities among adjacent frames to further compact the data. In such systems, differential encoding (DPCM) is used to transmit only the difference between an actual frame and a prediction of the actual frame. The prediction is based on information derived from a previous frame of the same video sequence. Examples of such systems can be found in U.S. Pat. No. 5,068,724 entitled "Adaptive Motion Compensation for Digital Television" and 5,057,916 entitled "Method and Apparatus for Refreshing Motion Compensated Sequential Video Images." A description of an HDTV broadcast system in which signals are transmitted in a compressed form is provided in W. Paik, "DigiCipher--All Digital, Channel Compatible, HDTV Broadcast System," IEEE Transactions on Broadcasting, Vol. 36, No. 4, December 1990, incorporated herein by reference.
Systems such as the HDTV broadcast system disclosed in the aforementioned Paik article transmit data in the form of variable length data packets ("data blocks") comprising variable length codewords (e.g., "Huffman" codewords). Since the data packets are of variable length, it is critical that the receiver have a means for distinguishing between adjacent packets. In other words, the receiver must keep track of when a current data packet ends and the next data packet starts. In the event that a transmission error occurs, which alters the expected length of a received data packet, or which causes an error in a packet length identifier transmitted with the data, synchronization at the receiver will be lost. It is important to provide a means for recovering from such transmission errors.
Often, error recovery is limited to the resynchronization of the receiver for each new video frame. By resynchronizing every frame, no more than one frame will be lost from the reconstructed video sequence. However, the reproduction of such an error in even one frame of a video image can result in a visible artifact that is unacceptable in a television picture. Concealment techniques, e.g., repeating a prior frame in lieu of a current frame in which the data has not been properly recovered, are known in the art. However, such concealment techniques will not always be effective in preventing noticeable degradations in a received video sequence.
It would be advantageous to provide a scheme for recovering from transmission errors more often than once per frame. Where date is transmitted in variable length packets, e.g., where successive blocks of quantized transform coefficients are transmitted, this would prevent an error in the length of one packet from propagating throughout the rest of the video frame. It would be still further advantageous to provide a scheme wherein each received data block is processed independently, such that the corruption of data in any one data block will not affect the processing of subsequent data blocks in the incoming data stream.
The present invention provides an error recovery scheme that enjoys the aforementioned advantages, and specifically facilitates the recovery from transmission errors within a received video frame, without waiting until a succeeding frame in order to correct the error or reacquire the video signal.